1. Field of the Invention
This invention relates to a system for measuring a distance to an object to be photographed by a camera.
2. Description of the Prior Art
A distance measuring system of the active type is conventionally employed for a distance measuring system for a camera. The distance measuring system of the active type is constructed such that a parallel beam of light is emitted in parallel to an optical axis of the camera from a light source provided in the camera, and an image of a spot on an object for photographing, upon which the light beam is irradiated, is formed such that it may extend over two light receiving elements disposed sidewardly of the light source in order to detect a distance to the object in accordance with a ratio between outputs of the two light receiving elements. In measuring a distance with the distance measuring system of the active type, since the position of an image formed on a light receiving plane by light which is emitted from a light emitting element and reflected from an object varies in accordance with a distance to the object, the distance to the object is measured in accordance with a ratio between outputs of light receiving elements with respect to the image. However, since the range of distances to the object to be photographed varies widely, the position range in which the image is formed varies. Accordingly, if it is attempted to measure a distance to an object which varies from 0 to .infin. while the light emitting element and the light receiving elements are kept fixed, then an image may not be formed on the light receiving elements where the distance is relatively short.
An exemplary conventional light measuring systems is illustrated in FIGS. 21a and 21b. Referring to FIGS. 21a and 21b, a beam of light projected from a light emitting element LED is reflected from an object for photographing and forms an image I on a pair of light receiving elements SPC1 and SPC2. Since the position of the image I on the light receiving elements SPC1 and SPC2 is differentiated in accordance with a distance to the object which may vary within a range from S1 to S5, a ratio between outputs of the light receiving elements SPC1 and SPC2 varies in accordance with the position of the image I. The relationship between the distance to the object and the ratio of the output of the light receiving element SPC2 to the output of the light receiving element SPC1 which varies in accordance with the distance is represented by such a relational curve as shown in FIG. 22. Since the ratio increases substantially, constantly and independently of a reflection factor of the object, the distance to the object can be measured by measuring the ratio. However, where the distance to the object is relatively short, as in the distance range S4 or S5 shown in FIG. 21a, the image I formed by reflected light from the object is displaced rightwardly in FIG. 21b so that it may be formed little on the plane of the light receiving element SPC1 but may be formed almost entirely on the other light receiving element SPC2. Consequently, the ratio between outputs of the light receiving elements SPC2 and SPC1 is no more changed in accordance with the distance to the object and thus approaches a saturated condition as seen in FIG. 22. Accordingly, it is a problem that the resolution in distance measurement of the distance measuring system is so deteriorated in a relatively short distance range that measurement of a distance may be disabled.